1. Field of the Invention
This invention relates to a holder incorporated in a working apparatus, an exposing apparatus or an inspecting apparatus used in relatively clean environment and vacuum which is used for mounting a substrate to be worked, a substrate to be exposed or a substrate to be inspected.
2. Related Background Art
Various holders of this kind have heretofore been proposed and put into practical use. Particularly in exposing apparatus used for the manufacture of semiconductor elements, liquid crystal display elements or the like, a holder of the vacuum mounting type has long been utilized to hold and flatten a photosensitive substrate to be exposed (a wafer or a glass plate). Some examples of the prior-art holders will hereinafter be described with reference to FIGS. 1A to 1D of the accompanying drawings.
FIG. 1A shows an example of the surface structure of a wafer holder WH1 of the oldest type, and the whole of this holder is made into a thick disc-like shape by the use of a metal, such as aluminum, or ceramics. An opening SU for center up is formed in the central portion of the holder WH1. A vertically movable center up member for raising a wafer placed thereon extends through the opening SU. The supporting surface of the wafer holder WH1 is polished so that the flatness thereof may become very high to hold the wafer flatly. Further, a plurality of (here, three) concentric vacuum grooves VC1, VC2 and VC3 are formed at suitable intervals in the supporting surface. The width and depth of these grooves are of the order of 1-2 mm. A groove VC4 for connecting the three grooves VC1, VC2 and VC3 radially thereof is formed at each suitable angle (here, 90.degree. ) in the circumferential direction. Intake holes connected to a vacuum source are formed at suitable locations in these grooves.
FIG. 1B shows the surface structure of a wafer holder WH2 conceived to eliminate the problem peculiar to the wafer holder of FIG. 1A, and as shown, a number of concentric grooves VCS are radially formed at a fine pitch in accordance with the configuration of a wafer, or a number of spiral grooves VCS are formed. The width and pitch of the grooves VCS are, for example, of the order of 1 mm and 2-3 mm, respectively. If the groove VCS are thus arranged at a fine pitch on the whole surface, the area of contact in which the protruded portion (supporting surface) between adjacent grooves contacts the back of the wafer becomes markedly smaller than in FIG. 1A. Therefore, the possibility of foreign substances (dust) being interposed between the supporting surface (protruded portion) and the back of the wafer becomes small, and even if foreign substances are interposed therebetween, the foreign substances can be expected to be eliminated toward the grooves VCS in the process of the wafer being vacuum mounted, because the grooves VCS lie at both sides of the protruded portion.
FIG. 1C shows the surface structure of a holder WH3 in which the area of contact with the back of the wafer is made still smaller than that in FIG. 1B. This holder WH3 has an annular protruded surface R1 matching the configuration of a wafer placed thereon, and an inner annular protruded surface R2 surrounding an opening SU for center up, and they are formed so as to protrude by the order of 1 to several mm relative to a concave portion BT. A plurality of pins PV each having a diameter of the order of 0.1-1 mm at the tip end thereof and contacting with the back of the wafer are two-dimensionally studded at substantially uniform intervals in the concave portion BT. Further, the tip ends of the protruded surfaces R1, R2 and pins PV of the holder WH3 are formed so as to be flush with one another at the order of 1 micron. Intake holes for vacuum application are formed in the concave portion BT, and the vacuum mounting of the wafer is effected by the whole of the interior of the concave portion BT being evacuated with the wafer placed on the holder.
FIG. 1D shows a portion of the surface of a holder on which, as in FIG. 1C, pin-like projections PV' are formed at uniform intervals and intake holes CC for evacuation are formed at the tip ends of the projections PV'. This holder has an advantage that because all of the numerous projections PV' create an adsorbing force, the annular protruded surfaces R1 and R2 for making the concave portion BT into a closed space as in FIG. 1C need not be provided.
As described above, improvements have historically been made toward a reduced area of contact with the back of the wafer. This is because when foreign substances such as dust are interposed between the supporting surface and the back of the wafer, that portion of the wafer surface (exposed surface) becomes bulged thereby and the flatness of the wafer is remarkably reduced. It is also because higher resolving power is required year after year of exposing apparatus used for the manufacture of semiconductor elements. In the case of the projection exposure system, the portion bulged by foreign substances causes an out-of-focus condition to thereby cause the inconvenience that however good the resolving power of the projection optical system may be, a pattern of a desired line width cannot be exposed or the accuracy of superposition is not enhanced.
From such circumstances, the area of contact is made as small as possible with a view to reduce the probability of foreign substances being interposed.
In common exposing apparatus for microlithography a pattern formed on a negative called a reticule is uniformly irradiated with illuminating light from a high luminance light source (such as a mercury lamp or a laser) and the light transmitted therethrough is imaged on the resist (sensitive agent) of a photosensitive substrate such as a wafer through a projection optical system to thereby effect the transfer of the pattern. At this time, the resist is applied to the surface of the substrate at a predetermined thickness (0.8-2 .mu.m) and a suitable exposure amount matching the transfer of the pattern is determined. The exposure amount is greatly varied depending on photosensitivity and in the reflectance of the substrate, even for a resist layer of the same thickness. In any case, most of exposing energy (chiefly ultraviolet ray) imparted to the photosensitive substrate is finally converted into thermal energy and diffused and accumulated in the substrate.
Particularly when a photosensitive substrate is to be exposed by the step and repeat system, even though the exposure time of one shot on a wafer exposed with one cycle of stepping is a short time (1 sec. or less), the exposing energy of the one shot is considerably high and amounts to 800 mW/cm.sup.2 or greater on the wafer. Therefore, as the exposure of a wafer to each shot progresses, the heat accumulation in the photosensitive substrate also progresses, and thus, thermal deformation of the substrate occurs.
The thermal deformation results in the partial expansion and contraction of the substrate, and means that the superposition error of a shot area formed there and a pattern image to be superposed and exposed thereon increases. This superposition error is of the order of 10--30 nm and has heretofore been neglected because the total overlay accuracy has not been so strict, but the error becomes significant when the line width comes into a submicron area. Particularly, in the process of 16 MD-RAM and so on, the line width is of the order of 0.5 .mu.m, and 50-70 nm or less is desired for the total overlay as well.
Accordingly, it becomes important to quickly exhaust the accumulated quantity of heat outwardly of the substrate in order to suppress the thermal deformation of the substrate. That is, it becomes necessary to cause the quantity of heat in the substrate to escape to the holder. For that purpose, it is also conceived to provide an active temperature control member such as a Peltier element within the holder or provide a structure for circulating temperature-controlled fluid in the holder, thereby quickly absorbing the quantity of heat in the substrate.
However, as previously described, the surface of the holder has been tendency to make the area of contact thereof with a substrate such as a wafer as small as possible, and there has been a disadvantage that the quantity of heat accumulated in the substrate is not quickly transferred to the holder side.